I recently had issues with a 150cc running way too lean. I changed 3 carbs and still too lean. I suspected a blown seal, cracked head, bad valve or other hardware failure to be the culprit. To eliminate the hard parts I had to make sure the timing was correct and to make sure I had no loss of compression with a gauge.....

I took some pics to help, but I used my cell phone and they are low quality...

To check the timing I removed the fan cover on the right side of the motor....


I removed the spark plug and inserted a small screwdriver and turned the motor the clockwise direction that it normally spins until I felt the piston at Top-Dead-Center. I then checked the timing marks (seen as the "T") with the metal arrow that is at the rear of the fan motor housing. The "T" lined up perfectly. This tells me that the crank timing is in line with Top-Dead-Center and that there is no issue there.

So now I need to check the valve timing. To do this I needed to pull the valve cover. Once the cover is off and you are at Top-Dead-Center with the "T" in the proper place on the fan motor flywheel(as shown in the last pic) go to the left side of the engine and check the marks on the timing chain gear.....


The lines on the gear should be parallel to the cylinder head as shown in the image. Pay special attention that one mark has a hole above it and the other mark has a plugged hole. It is important that the mark with the hole be at the top as shown in the picture. This indicates that the timing for the valves matches the timing for the crank and that there is NO timing issue.

The next step is turn the motor by hand until the valves close. You can then slide a .005 gauge under the valve to check clearances. It should be snug but not loose and not too tight that you fight the gauge. Check both valves. If the .005 gauge fits between the rocker arm and the valve stem then your clearances are also fine. If not tighten or loosen the bolt as needed.

Once these items were ruled out and shown to be operating properly I threaded a 10mm adapter onto a compression gauge and cranked the motor....


The gauge held at 130psi and never lost pressure after 10 minutes. This proved that there is NOT an air leak, blown gasket, or other air leak issue with the cylinder and that compression is indeed good. If the gauge is leaking down, and you passed the above tests than a head gasket would of been likely. Other problems could be a bad valve, bad rings, or a cracked head. Disassemble the motor to determine the actual problem.

I also replaced all gaskets to my aluminum intake pipe both at the cylinder and the carb, and I tried with and without factory air box. I am still having trouble and am downsizing my carb to see if I can control the lean issue better.

I hope this helps if people want to check there timing.